The invention relates to a process for pressure welding two metal bars together by their ends, and its use in a process for continuously producing copper wire rods in an in-line plant with wire bars as starting material.
As is well-known, copper wire rods can be produced by continuous casting on a casting wheel into a continuous string which is then passed through a continuous rolling mill. The difficulty is that this system is not flexible with respect to short interruptions and changes of production speeds and changes of composition of the copper alloys. Its best use is for a continuous basic production of a given alloy composition.
However, conventional production of wire rods, in which the conventional wire bars as starting material are gradually rolled into the desired cross-section, still has the important drawback that it is a discontinuous method. This means that the system delivers one by one a number of limited lengths of wire rods which must be welded to each other into conventional coil lengths and these welds are the weak points in further wire drawing.
It is for that reason that we, in previous attempts, have tried out an in-line method, as well-known in the metal industry, for application in a process for continuously producing copper wire rods, in which process the wire bars are firstly passed at hot rolling temperature through a rolling mill for delivering one after another a number of copper bars, in which the leading end of each copper bar leaving the rolling mill is welded to the trailing end of continuous string formed by the preceding bars which are welded to each other, and where finally this continuous string is continuously rolled through a continuous rolling mill into wire rods. The output of this system produces an unlimited length of wire rods where the welds, made in an intermediate stage, have been rolled out and recrystallized and which were expected to form a better joint. The copper bars, as they left the initial rolling mill, were still at rolling temperature and, whilst at that temperature, the ends of the bars were pressed into each other over a certain distance whilst these ends were further heated up by means of an electric current passed through the contact surface.
Unfortunately, this method appeared not to be practicable for two reasons. A first reason relates to the necessary time to produce the weld. The ends of the copper bars are oxidised, even when they have been cut off just before welding in order to provide fresh unoxidised copper material for contact with the other end. The bars are hot indeed and the extremities directly oxidise again before pressing. For that reason the ends are pressed into each other over a distance, in general three to five times the diameter of the bars, sufficient to ensure that substantially all of the oxidised initial contact surface material flows outward, out of the cross-sectional shape of the bars, to form a ring around the weld, which weld is then formed inside said cross-sectional shape between fresh unexposed copper material coming from inside the bar. But this means then that the gripper of the copper bars must keep these bars so that a length of three to five times the diameter protrudes out of the gripper before pressing. When both protruding ends are then applied to each other for pressing, this results in a total length of six to ten times the bar diameter, of a very weak heated copper material, put under axial pressure. The result is that this length tends to buckle instead of flaring out and expelling the unclean surface material outward. In order to avoid this, as is known in the art, one must have recourse to a pressing operation in a plurality of steps. The grippers take the ends so as to only protrude by up to one and a half times the diameter, the ends are pressed into each other over a length of up to once the diameter, then the grippers release the extremities and grip them again at a point a bit further from the joint press again both extremities further into each other over a further length and so on until a sufficient total length of both extremities are pressed into each other to be sure that no substantial amount of the oxidised surface is remaining inside the cross-sectional shape of the copper bars. Thus no more than three times the diameter of the bars has then ever been exposed to buckling load.
But a practical system for delivering up to 20 tons of wire rods per hour must be able to treat one wire bar, or to conduct one welding cycle, every 22 seconds. Such a cycle requires at least the steps of gripping both ends and bolting both grippers, pressing both ends against each other, unbolting both grippers and releasing both ends, and evacuation and translation of the bars into the position for the next weld. So, it is a practical inevitable necessity that the weld is conducted by a single pressing operation and not by two or more successive pressing steps. However, only in this latter case, is the danger of buckling sufficiently removed.
The second difficulty which makes the method hardly practicable relates to the quality of the weld and of the joint after the weld has been rolled out in the next continuous rolling operation. It appears to comprise a number of microcracks caused by rolling the welding zone.